It is known in the art of LCD compensation that the phase retardation of light varies according to wavelength (λ), causing color shift. This wavelength dependence (or dispersion) characteristic of the compensation film may be taken into account when designing an LCD device so that color shift is reduced. Wavelength dispersion curves are defined as “proper” or “reversed” with respect to a compensation film with positive or negative retardance. A compensation film with positive retardance (positive C-plate) has a proper curve in which the value of phase retardation is increasingly positive toward shorter wavelengths, and a reversed curve in which the value of phase retardation is decreasingly positive toward shorter wavelengths. A compensation film with negative retardance (negative C-plate) has a proper curve in which the value of phase retardation is increasingly negative toward shorter wavelengths, and a reversed curve in which the value of phase retardation is decreasingly negative toward shorter wavelengths. Exemplary shapes of these curves are shown in FIG. 1.
In the dark (off) state, vertically aligned (VA) LC cells have a wavelength dispersion curve similar to a proper curve for positive retardation. Since there is basically no light passing through the display in the dark state, VA-LC cells only require compensation for retardation and not for dispersion. Accordingly, VA-LC cells are typically compensated by a negative C-plate to prevent light leakage and improve the contrast of the image. On the other hand, in the lighted (on) state, the VA-LC cell has a wavelength dispersion curve similar to a proper curve for negative retardation, which leads to color distortion due to wavelength dispersion of its proper retardation curve. Thus, VA-LC cells in the lighted state require compensation for both retardation and dispersion. Accordingly, a positive C-plate may be used to compensate color distortion in the lighted state.
Negative C-plates, such as polyimide, have a negative birefringence value throughout the wavelength range of 400 nm<λ<800 nm. Negative C-plates typically have a proper wavelength dispersion curve with increasingly large negative retardation value toward shorter wavelengths. As such, negative C-plates can compensate for the dark (off) VA-LC cell's retardation to reduce the light leakage and thus increase the image contrast. However, negative C-plates with proper wavelength dispersion do not compensate for the lighted VA-LC cell's retardation or dispersion, and, as such, color shift and poor image quality are not alleviated at those shorter wavelengths and may even increase dispersion in the lighted (on) state. Thus, traditional negative C-plates that have a proper dispersion curve alone typically do not have the required wavelength dispersion characteristic to eliminate color shift, especially at shorter wavelengths.
Multilayer compensation films have been proposed to compensate for VA-LCDs. U.S. Patent Application No. 2005/0057714 discloses a complex light compensation C-plate with at least two C-plates having different dispersion ratios.
When both the dark and lighted states are taken into consideration, a compensation film with negative retardation and a reversed wavelength dispersion curve may provide the best compensation for a VA-LC cell. Accordingly, there remains a need in the LCD industry for an improved optical compensation film having retardance and wavelength dispersion characteristics that improve both light leakage in the dark state and color distortion in the lighted state of VA-LCD devices.